Native Client

Dynamic Linking and Loading with glibc

Introduction

This document describes how to create and deploy dynamically linked and loaded applications with the glibc library in the Native Client SDK. Before reading this document, we recommend reading Building Native Client Modules

C standard libraries: glibc and newlib

The Native Client SDK comes with two C standard libraries — glibc and newlib. These libraries are described in the table below.

Library Linking License Description
glibc dynamic or static GNU Lesser General Public License (LGPL) glibc is the GNU implementation of the POSIX standard runtime library for the C programming language. Designed for portability and performance, glibc is one of the most popular implementations of the C library. It is comprised of a set of interdependent libraries including libc, libpthreads, libdl, and others. For documentation, FAQs, and additional information about glibc, see GLIBC
newlib static Berkeley Software Distribution (BSD) type free software licenses newlib is a C library intended for use in embedded systems. Like glibc, newlib is a conglomeration of several library parts. It is available for use under BSD-type free software licenses, which generally makes it more suitable to link statically in commercial, closed-source applications. For documentation, FAQs, and additional information about newlib, see the newlib documentation.

For proprietary (closed-source) applications, your options are to either statically link to newlib, or dynamically link to glibc. We recommend dynamically linking to glibc, for a couple of reasons:

  • The glibc library is widely distributed (it’s included in Linux distributions), and as such it’s mature, hardened, and feature-rich. Your code is more likely to compile out-of-the-box with glibc.
  • Dynamic loading can provide a big performance benefit for your application if you can structure the application to defer loading of code that’s not needed for initial interaction with the user. It takes some work to put such code in shared libraries and to load the libraries at runtime, but the payoff is usually worth it. In future releases, Chrome may also support caching of common dynamically linked libraries such as libc.so between applications. This could significantly reduce download size and provide a further potential performance benefit (for example, the hello_world example would only require downloading a .nexe file that’s on the order of 30KB, rather than a .nexe file and several libraries, which are on the order of 1.5MB).

Native Client support for dynamic linking and loading is based on glibc. Thus, if your Native Client application must dynamically link and load code (e.g., due to licensing considerations), we recommend that you use the glibc library.

SDK toolchains

The Native Client SDK contains multiple toolchains, which are differentiated by target architecture and C library:

Target architecture C library Toolchain directory
x86 newlib toolchain/<platform>_x86_newlib
x86 glibc toolchain/<platform>_x86_glibc
ARM newlib toolchain/<platform>_arm_newlib
PNaCl newlib toolchain/<platform>_pnacl

In the directories listed above, <platform> is the platform of your development machine (i.e., win, mac, or linux). For example, in the Windows SDK, the x86 toolchain that uses glibc is in toolchain/win_x86_glibc.

To use the glibc library and dynamic linking in your application, you must use a glibc toolchain. (Currently the only glibc toolchain is <platform>_x86_glibc.) Note that you must build all code in your application with one toolchain. Code from multiple toolchains cannot be mixed.

Specifying and delivering shared libraries

One significant difference between newlib and glibc applications is that glibc applications must explicitly list and deploy the shared libraries that they use.

In a desktop environment, when the user launches a dynamically linked application, the operating system’s program loader determines the set of libraries the application requires by reading explicit inter-module dependencies from executable file headers, and loads the required libraries into the address space of the application process. Typically the required libraries will have been installed on the system as a part of the application’s installation process. Often the desktop application developer doesn’t know or think about the libraries that are required by an application, as those details are taken care of by the user’s operating system.

In the Native Client sandbox, dynamic linking can’t rely in the same way on the operating system or the local file system. Instead, the application developer must identify the set of libraries that are required by an application, list those libraries in a Native Client manifest file, and deploy the libraries along with the application. Instructions for how to build a dynamically linked Native Client application, generate a Native Client manifest (.nmf) file, and deploy an application are provided below.

Building a dynamically linked application

A dynamically linked application typically includes one Native Client module and one or more shared libraries. (How to allocate code between Native Client modules and shared libraries is a question of application design that is beyond the scope of this document.) Each Native Client module and shared library must be compiled for at least the x86 32-bit and 64-bit architectures.

The dlopen example in the SDK

The Native Client SDK includes an example that demonstrates how to build a shared library, and how to use the dlopen() interface to load that library at runtime (after the application is already running). Many applications load and link shared libraries at launch rather than at runtime, and hence do not use the dlopen() interface. The SDK example is nevertheless instructive, as it demonstrates how to build Native Client modules (.nexe files) and shared libraries (.so files) with the x86 glibc toolchain, and how to generate a Native Client manifest file for glibc applications.

The SDK example, located in the directory examples/dlopen, includes two C++ files:

eightball.cc
This file implements the function Magic8Ball(), which is used to provide whimsical answers to user questions. The file is compiled into a shared library, libeightball.so.
dlopen.cc
This file implements the Native Client module, which loads libeightball.so, receives messages from JavaScript (sent in response to user input), calls Magic8Ball() to generate answers, and sends messages back to JavaScript with the generated answers. The file is compiled into a .nexe file.

Run make in the dlopen directory to see the commands the Makefile executes to build x86 32-bit and 64-bit .nexe and .so files, and to generate a .nmf file. These commands are described below.

Building a Native Client module (.nexe file)

The Makefile in the dlopen example builds dlopen.cc into a .nexe file using the two commands shown below. (For simplicity, the full path to the compiler/linker is not shown; the tool is located in the bin directory in the x86 glibc toolchain, e.g. toolchain/win_x86_glibc/bin.)

To compile dlopen.cc into dlopen_x86_32.o:

i686-nacl-g++ -o dlopen_x86_32.o -c dlopen.cc -m32 -g -O0 -pthread -std=gnu++98 -Wno-long-long -Wall

To link dlopen_x86_32.o into dlopen_x86_32.nexe:

i686-nacl-g++ -o dlopen_x86_32.nexe dlopen_x86_32.o -m32 -g -ldl -lppapi_cpp -lppapi

A few of the flags in these commands are described below:

-o file
put the output in file
-c
compile the source file, but do not link it
-m32
produce 32-bit code (i.e., code for the x86-32 target architecture)
-g
produce debugging information
-O0
use a base optimization level that minimizes compile time
-pthread
support multithreading with the pthread library
-W warning
request or supress the specified warning
-l library
use the specified library when linking (per C library naming conventions, the linker uses the file lib*library*.so, or if that file is not available, lib*library*.a; e.g., -ldl corresponds to libdl.so or libdl.a)

Many of these flags are optional; you need not use all of them to compile and link your application. For example, you only need to use -ldl if your application uses the dlopen() interface to open a library at runtime. The toolchains in the Native Client SDK are based on the gcc compiler; see gcc command options for a full description of the gcc flags. For flags that are recommended with Native Client, see compile flags for different development scenarios.

Note that you can combine the compile and link steps to build a .nexe file using one command. Simply run i686-nacl-g++ once and use the appropriate combination of flags (omit the -c flag and include the -l flag with the required libraries):

i686-nacl-g++ -o dlopen_x86_32.nexe dlopen.cc ^ -m32 -g -O0 -pthread -std=gnu++98 -Wno-long-long -Wall  -ldl -lppapi_cpp -lppapi

(The carat ^ allows the command to span multiple lines on Windows; to do the same on Mac and Linux use a backslash instead. Or you can simply type the command and all its arguments on one line.)

The commands above build a 32-bit .nexe. To build a 64-bit .nexe, run the same commands but with the -m64 flag instead of -m32, and of course specify different output file names. Check the Makefile in the dlopen example to see the set of commands that is used to generate 32-bit and 64-bit .nexes.

Building a shared library (.so file)

The Makefile in the dlopen example builds eightball.cc into a .so file using the two commands shown below.

To compile eightball.cc into eightball_x86_32.o:

i686-nacl-g++ -o eightball_x86_32.o -c eightball.cc -m32 -g -O0 -pthread -std=gnu++98 -Wno-long-long -Wall -fPIC

To link eightball_x86_32.o into eightball_x86_32.so:

i686-nacl-g++ -o libeightball.so eightball_x86_32.o -m32 -g -ldl -lppapi_cpp -lppapi -shared

A couple of the important flags in these commands are described below:

-fPIC
generate position-independent code (PIC) suitable for use in a shared library (this flag is required for all x86 64-bit modules and for 32-bit shared libraries)
-shared
produce a shared object that can be linked with other objects to form an executable (this flag is required for .so files) As when building a .nexe, you can combine compiling and linking into one step by running i686-nacl-g++ once with the appropriate combination of flags.

As with .nexes, you need to generate both 32-bit and 64-bit versions of a shared object – see the dlopen example for an illustration. In the dlopen example, the shared objects are put into the subdirectories lib32 and lib64. These directories are used to collect all the shared libraries needed by the application, as discussed below.

Generating a Native Client manifest file for a dynamically linked application

The Native Client manifest file must specify the full list of executable files needed by an application, including the recursive closure of shared library dependencies. Take a look at the manifest file in the dlopen example to see how a glibc-style manifest file is structured. (Run make in the dlopen directory to generate the manifest file if you haven’t done so already.) Here is an excerpt from dlopen.nmf:

{
  "files": {
    "libeightball.so": {
      "x86-64": {
        "url": "lib64/libeightball.so"
      },
      "x86-32": {
        "url": "lib32/libeightball.so"
      }
    },
    "libstdc++.so.6": {
      "x86-64": {
        "url": "lib64/libstdc++.so.6"
      },
      "x86-32": {
        "url": "lib32/libstdc++.so.6"
      }
    },
    "libppapi_cpp.so": {
      "x86-64": {
        "url": "lib64/libppapi_cpp.so"
      },
      "x86-32": {
        "url": "lib32/libppapi_cpp.so"
      }
    },
... etc.

In most cases, you can use the create_nmf.py script in the SDK to generate a manifest file for your application. The script is located in the tools directory (e.g., pepper_28/tools).

The Makefile in the dlopen example generates the manifest file dlopen.nmf by running the following command:

python <NACL_SDK_ROOT>/tools/create_nmf.py ^
  -D <NACL_SDK_ROOT>/toolchain/win_x86_glibc/x86_64-nacl/bin/objdump ^
  -o dlopen.nmf ^
  -s . ^
  dlopen_x86_32.nexe dlopen_x86_64.nexe lib32/libeightball.so lib64/libeightball.so ^
  -L <NACL_SDK_ROOT>/toolchain/win_x86_glibc/x86_64-nacl/lib32 ^
  -L <NACL_SDK_ROOT>/toolchain/win_x86_glibc/x86_64-nacl/lib64

(The carat ^ allows the command to span multiple lines on Windows; to do the same on Mac and Linux use a backslash instead, or you can simply type the command and all its arguments on one line. <NACL_SDK_ROOT> represents the path to the top-level directory of the bundle you are using, e.g., <location-where-you-installed-the-SDK>/pepper_28.)

Run python create_nmf.py --help to see a description of the command-line flags. A few of the important flags are described below.

-D tool
use tool to read information about a file and determine shared library dependencies (the tool must be a version of the objdump utility)
-s directory
use directory to stage libraries (libraries are added to lib32 and lib64 subfolders)
-L directory
add directory to the library search path

As an alternative to using create_nmf.py, you can also chase down the full list of shared library dependencies manually and add those to your .nmf file. To do so, start by running the Native Client version of the objdump utility on your .nexe file, as shown below. (The objdump utility is located in the same directory as the glibc toolchain, e.g., toolchain/win_x86_glibc/bin.)

i686-nacl-objdump -p dlopen_x86_32.nexe

A .nexe file contains compiled machine code, as well as headers that describe the contents of the file and information about how to use the file. The objdump utility lets you examine the file’s headers, including the “Dynamic Section,” which specifies shared library dependencies, as in this example output from the dlopen example:

Dynamic Section:
  NEEDED               libdl.so.32d9fc17
  NEEDED               libppapi_cpp.so
  NEEDED               libpthread.so.32d9fc17
  NEEDED               libstdc++.so.6
  NEEDED               libm.so.32d9fc17
  NEEDED               libgcc_s.so.1
  NEEDED               libc.so.32d9fc17
  INIT                 0x01000140
  FINI                 0x01002560
  HASH                 0x110025fc
  ...

All the files that are identified as NEEDED in the “Dynamic Section” portion of the objdump output are files that you need to list in your Native Client manifest file and distribute with your application. (The numbers listed at the end of the file names are version numbers, and you must list and distribute those exact versions.) Once you’ve identified the shared libraries that are needed by your .nexe file, you must repeat the process recursively: Run objdump on each of the NEEDED files, and add the newly-identified NEEDED files to your manifest file and to your distribution directories. To get the full list of libraries for an application, repeat the process until you’ve identified the recursive closure of dependencies.

Deploying a dynamically linked application

As described above, an application’s manifest file must explicitly list all the executable code modules that the application requires, including modules from the application itself (.nexe and .so files), modules from the Native Client SDK (e.g., libppapi_cpp.so), and perhaps also modules from naclports or from middleware systems that the application uses. You must provide all of those modules as part of the application deployment process.

As explained in Distributing Your Application, there are two basic ways to deploy an application:

  • hosted application: all modules are hosted together on a web server of your choice
  • packaged application: all modules are packaged into one file, hosted in the Chrome Web Store, and downloaded to the user’s machine

You must deploy all the modules listed in your application’s manifest file for either the hosted application or the packaged application case. For hosted applications, you must upload the modules to your web server. For packaged applications, you must include the modules in the application’s Chrome Web Store .crx file. Modules should use URLs/names that are consistent with those in the Native Client manifest file, and be named relative to the location of the manifest file. Remember that some of the libraries named in the manifest file may be located in directories you specified with the -L option to create_nmf.py. You are free to rename/rearrange files and directories referenced by the Native Client manifest file, so long as the modules are available in the locations indicated by the manifest file. If you move or rename modules, it may be easier to re-run create_nmf.py to generate a new manifest file rather than edit the original manifest file. For hosted applications, you can check for name mismatches during testing by watching the request log of the web server hosting your test deployment.

Opening a shared library at runtime

Native Client supports a version of the POSIX standard dlopen() interface for opening libraries explicitly, after an application is already running. Calling dlopen() may cause a library download to occur, and automatically loads all libraries that are required by the named library.

The best practice for opening libraries with dlopen() is to use a worker thread to pre-load libraries asynchronously during initialization of your application, so that the libraries are available when they’re needed. You can call dlopen() a second time when you need to use a library – per the specification, subsequent calls to dlopen() return a handle to the previously loaded library. Note that you should only call dlclose() to close a library when you no longer need the library; otherwise, subsequent calls to dlopen() could cause the library to be fetched again.

The dlopen example in the SDK demonstrates how to open a shared library, magiceightball.so, at runtime. To reiterate, the example includes two C++ files:

  • eightball.cc: this is the shared library that implements the function Magic8Ball() (this file is compiled into libeightball.so)
  • dlopen.cc: this is the Native Client module that loads libeightball.so and calls Magic8Ball() to generate answers (this file is compiled into dlopen_x86_{32,64}.nexe)

When the Native Client module starts, it kicks off a worker thread that calls dlopen() to load magiceightball.so. When the download of libeightball.so completes, the worker thread schedules a callback function on the main thread. The callback function calls dlopen() for magiceightball.so a second time; this second call obtains a proper handle to the library. Once the module has a handle to the library, it grabs the entry point in libeightball.so for the Magic8Ball() function. When a user types in a query and clicks the ‘ASK!’ button, the module calls Magic8Ball() to generate an answer, and returns the result to the user.

The sequence of calls in the dlopen module is illustrated by the pseudo-code in the table below:

Worker Thread Main Thread
pthread_create(.., LoadLibrariesOnWorker, ..)
-
-
-
-
LoadDone()
  UseLibrary()
    dlopen("libeightball.so", ...)
    offset = dlsym(..., "Magic8Ball")
HandleMessage()
  _eightball = (TYPE_eightball) offset;
  PostMessage()
-
LoadLibrariesOnWorker()
  LoadLibrary()
    dlopen("libeightball.so",...)
    CallOnMainThread(.., LoadDone, ..)
-
-
-
-
-
-
-

Troubleshooting

If your .nexe isn’t loading, the best place to look for information that can help you troubleshoot the problem is stdout and nacllog. See the Debugging page for instructions about how to access those streams.

Here are a few common error messages and explanations of what they mean:

/main.nexe: error while loading shared libraries: /main.nexe: failed to allocate code and data space for executable
The .nexe may not have been compiled correctly (e.g., the .nexe may be statically linked). Try cleaning and recompiling with the glibc toolchain.
/main.nexe: error while loading shared libraries: libpthread.so.xxxx: cannot open shared object file: Permission denied
(xxxx is a version number, for example, 5055067a.) This error can result from having the wrong path in the .nmf file. Double-check that the path in the .nmf file is correct.
/main.nexe: error while loading shared libraries: /main.nexe: cannot open shared object file: No such file or directory
If there are no obvious problems with your main.nexe entry in the .nmf file, check where main.nexe is being requested from. Use Chrome’s Developer Tools: Click the menu icon menu-icon, select Tools > Developer Tools, click the Network tab, and look at the path in the Name column.
NaCl module load failed: ELF executable text/rodata segment has wrong starting address
This error happens when using a newlib-style .nmf file instead of a glibc-style .nmf file. Make sure you build your application with the glic toolchain, and use the create_nmf.py script to generate your .nmf file.
NativeClient: NaCl module load failed: Nexe crashed during startup
This error message indicates that a module crashed while being loaded. You can determine which module crashed by looking at the Network tab in Chrome’s Developer Tools (see above). The module that crashed will be the last one that was loaded.
/lib/main.nexe: error while loading shared libraries: /lib/main.nexe: only ET_DYN and ET_EXEC can be loaded
This error message indicates that there is an error with the .so files listed in the .nmf file – either the files are the wrong type or kind, or an expected library is missing.
undefined reference to ‘dlopen’ collect2: ld returned 1 exit status
This is a linker ordering problem that usually results from improper ordering of command line flags when linking. Reconfigure your command line string to list libraries after the -o flag.

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